1. Field of the Invention
The present invention is directed to an electric machine with a separately excited rotor order for the generation of magnetic fields which follow one another in the movement direction of the rotor, each magnetic field corresponding to a whole magnetic period, and with stator poles which are arranged so as to follow one another along the rotor arrangement.
2. Description of the Related Art
In known electric machines of this type with a separately excited rotor (or stator, the two terms rotor and stator being interchangeable for present purposes), the magnetic fields corresponding to the whole magnetic periods are formed by individual poles, a north pole and a south pole (for each magnetic period), which are separated from one another. Stator pole sets which correspond with respect to the arrangement and quantity of stator poles are provided in accordance with the quantity of magnetic fields formed by the pairs of rotor poles, so that corresponding arrangements of stator poles are located under every pair of rotor poles. Permanent magnets can be used, for example, for the rotor poles.
As a result of stray flux and especially owing to the gaps between the rotor poles, a rotor field is formed in machines of this type such that a movement of the rotor relative to the stator results in a nonlinear variation of the forces present at the rotor poles, which forces are dependent on the sum of the prevailing field amounts at the poles. This results finally in a self-locking behavior, i.e., a self-holding torque occurs (also known as cogging torque or cogging). This self-locking behavior is especially pronounced during slow idling. This effect occurs especially because the nonlinearly changing forces add up over the entire length of the stator or rotor due to the constantly identical ratios within the individual magnetic periods of the rotor field. In connection with the self-locking behavior, such machines have other disadvantages such as rough running, vibrations, the need to operate the machines as minimum speeds, etc.
Since narrow limits are imposed on variations in the shape of the stator and rotor poles, a reduction of the self-holding torque is possible only by means of offsetting the pole stack or laminated core, increasing the air gap, or by means of a specially developed pole head shape. However, these steps impair the operating parameters of the machine, so that a machine with a self-locking behavior which is improved as a result of these steps represents an unfavorable compromise.